The invention relates to a drill bit for drilling rock, having an elongated shank and at least one cutting plate at one end, the cutting plate having at least one end cutting edge running from the periphery to the rotary axis of the drill. The cutting edge is formed with the formation of a wedge angle xcex3 of an adjoining flank and rake face, the flank enclosing a clearance angle xcex2 and the rake face enclosing a rake angle xcex1 with a plane E perpendicular to the rotary axis D, and the wedge angle xcex3=180xc2x0xe2x88x92(xcex1+xcex2)
Drills for drilling rock generally consist of a cylindrical steel shaft formed with helically twisted flutes for the disposal of the drillings, a chucking part or shank end for attachment in the drill, and a drill bit head at the other end. The drill bit head is formed by one or more cutting plates connected to the steel shaft, as a rule by brazing, and made of a wear-resistant material such as hardmetal.
Such drill bits are required to have as high a drilling capacity (drilling progress per unit of time) as possible, on the one hand, and as long a tool life as possible, on the other hand.
The end of the tool life of the drill bit is generally determined by the maximum permissible width of the wear mark along the cutting edge at the rake faces and flanks of the end cutting edges. The maximum permissible width of the wear mark is about ⅔ of the thickness of the cutting plate.
The wear mark results from roughening of the rake faces and flanks on account of friction with the removed material particles. In addition, high temperatures occur in the cutting-edge region due to the friction and lead to a diffusion of the metal particles from the cutting-edge region into the removed material particles. As a result, the cutting edge is weakened and, if the maximum permissible width of the wear mark is exceeded, failure of the cutting plate rapidly occurs.
In addition, if the maximum permissible width of the wear mark is exceeded and if the inadmissible dulling of the cutting edge associated therewith occurs, the material is only removed to a greatly reduced extent. The energy introduced by the drilling tool is increasingly converted into heat by the increased friction, so that the drill bit may heat up to the temperature at which the brazing filler metal with which the cutting plate is soldered in place melts. A loss of brazing of the cutting plate and thus failure of the drill bit is the result.
Since the cutting speeds in the peripheral region of the drill bit are markedly higher than in the center region, in particular the cutting-edge regions at the periphery of the cutting plate are subjected to increased wear.
The most widespread conventional drills for drilling rock have, as drill bit head, a cutting plate having two end cutting edges which adjoin one another in a roof shape and in each case have a constant clearance angle xcex2 and wedge angle xcex3 along their profile from the periphery to the center of the drill bit.
In this case, the clearance angle xcex2is normally within a range of between 20xc2x0 and 30xc2x0, whereas the rake angle is in the order of magnitude of about 60xc2x0, so that a constant wedge angle of between 90xc2x0 and 100xc2x0 is obtained.
In these conventional drills having constant clearance angles and wedge angles over the length of the end cutting edges, the end of the tool life is determined by the maximum permissible width of the wear mark at the peripheral region of the cutting plate. The maximum permissible width of the wear mark, on account of the higher cutting speed in this region, is reached much earlier than in the center region. The latter, of course, could actually continue to be used.
German Utility Model DE 298 19 388 U1 describes a hard metal cutting plate for use on a rock drill bit, in which cutting plate the clearance angle of the end cutting edge decreases with increasing distance from the rotary axis of the drill bit. Due to that construction, the support of the cutting edge, in particular at the peripheral region of the drill bit, is certainly improved and thus the risk of fracture of the cutting plate is reduced.
On account of the large wedge angle, however, the maximum permissible width of the wear mark is reached relatively quickly and the drilling capacity which can be achieved is not very high.
The object of the present invention is to provide a drill bit for drilling rock which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which has a high drilling capacity and in which the period until the maximum permissible width of the wear mark is reached, in particular at the peripheral regions of the end cutting edges, is increased.
With the above and other objects in view there is provided, in accordance with the invention, a drill bit for drilling rock, comprising:
an elongated shaft having a forward end and defining a rotary axis of the drill bit;
at least one cutting plate disposed at the forward end of the elongated shaft, the cutting plate being formed with a rake face and a flank enclosing a wedge angle xcex3 therebetween and defining a cutting edge extending from a periphery to the rotary axis of the drill bit;
the flank enclosing a clearance angle xcex2 with a plane extending perpendicular to the rotary axis and the rake face enclosing a rake angle xcex1 with the plane;
wherein xcex1+xcex2+xcex3=180xc2x0, and the clearance angle xcex2 increases, at least in segments thereof, with an increasing distance from the rotary axis, and the wedge angle xcex3 decreases, at least in segments thereof, with an increasing distance from the rotary axis.
In other words, the objects of the invention are satisfied in that the clearance angle xcex2 of the cutting edge, at least in sections, increases with increasing distance from the rotary axis D and the wedge angle xcex3, at least in sections, decreases with increasing distance from the rotary axis D.
This achieves the effect that good support of the drill bit point is achieved by a large wedge angle xcex3 in the region of the rotary axis D of the drill, whereas the smaller wedge angle xcex3 in the region of the drill bit periphery ensures that the maximum permissible width of the wear mark, compared with prior art drills having a constant clearance angle xcex2 and wedge angle xcex3, is not reached until later, so that the wear in the center and in the peripheral region is made more uniform and thus the tool life is increased.
In accordance with an added feature of the invention, the clearance angle xcex2 increases continuously and the wedge angle xcex3 decreases continuously. In this way, the cutting edge is subjected to uniform stress. No load peaks occur, which could cause premature failure of the cutting edge by fracture.
In accordance with an additional feature of the invention, the clearance angle xcex2 increases from approximately 30xc2x0 at the rotary axis D to approximately 40xc2x0 at the periphery of the drill bit, and the wedge angle xcex3 decreases from approximately 90xc2x0 at the rotary axis to approximately 50xc2x0 at the periphery of the drill bit.
In accordance with another feature of the invention, the cutting plate is a single cutting plate formed with two straight cutting edges enclosing a point angle xcex4 of about 130xc2x0 with one another.
The configuration according to the invention is used especially frequently and advantageously in drills which are provided with a single cutting plate with two straight end cutting edges that enclose the point angle xcex4 of about 130xc2x0.
In practice, it has proved to be especially advantageous for a uniform high cutting capacity to provide a clearance angle xcex2 of about 30xc2x0 and a wedge angle xcex3 of about 90xc2x0 in the region of the rotary axis D, and to provide a clearance angle xcex2 of about 40xc2x0 and a wedge angle xcex3 of about 50xc2x0 in the peripheral region of the drill.
In accordance with a concomitant feature of the invention, each of the cutting edges is formed in two parts including a straight center segment and a straight peripheral segment, the peripheral segment jutting out above the center segment in a forward drilling direction and extending at an inclination substantially identical to an inclination of the center segment.
This is an especially advantageous variant of the drill bit according to the invention. It achieves the effect that that region of the cutting edge which is subjected to the greatest stress in terms of wear is deliberately strengthened by additional material build-up.
With the above and other objects in view there is also provided, in accordance with the invention, a drill bit cutting plate for rock drill bits which has the characteristics outlined above, namely:
a cutting edge with a wedge angle xcex3 defined by a rake face and a flank; the flank enclosing a clearance angle xcex2 with a plane extending orthogonally to an intended axis of rotation of the cutting plate and the rake face enclosing a rake angle xcex1 with the plane; wherein the angles xcex1+xcex2+xcex3 are complementary to sum to 180xc2x0, and the clearance angle xcex2 increases, at least in segments thereof, with an increasing distance from the rotary center, and the wedge angle xcex3 decreases, at least in segments thereof, with an increasing distance from the rotary center.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a drill bit for drilling rock, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.